In a radio communication system such as a cellular system, a reference signal for obtaining various indexes of a propagation channel and a transmission signal is introduced. Such a reference signal (RS) is also used, for example, in LTE (Long Term Evolution) for a next generation communication system studied in 3GPP (3rd Generation Partnership Project) which is an international standards body for mobile communication. In downlink communication from a base station to a user equipment, a reference signal which is transmitted from the transmission apparatus (base station) to the reception apparatus (user equipment) is used as principal uses in (1) estimation of a propagation channel for demodulation, (2) a quality measurement for the frequency scheduling or the adaptive MCS (Modulation and Coding Scheme) control, or the like. In LTE, in a multi-antenna system for applying MIMO (Multiple Input Multiple Output), a reference signal is transmitted in a predetermined radio resource unit.
In LTE-advanced (hereinafter, referred to as LTE-A) which is a communication system that advances LTE, in order to achieve further sophistication, introduction of high-order MIMO (for example, transmission 8 antenna) or cooperative multipoint transmission/reception (CoMP) is studied. In addition to the reference signal (first reference signal) which is studied in LTE, therefore, an additional reference signal (second reference signal) is necessary for LTE-A, and a method of transmission is discussed.
As shown in Non-patent Literature 1, for example, two kinds of reference signals respectively for the above-described uses are studied in LTE-A.
(1) Demodulation RS: one for PDSCH (Physical downlink shared channel) demodulation, to which the same layer number as that of PDSCH and Precoding are applied, and specific to User Equipment (UE) (UE-specific).
(2) CSI-RS: one for CSI (Channel State Information) observation, (as CSI, there are CQI (Channel Quality Indicator), PMI (Precoding Matrix Indicator), RI (Rank Indicator), and the like), to which Precoding is not applied, and specific to a cell (cell-specific).
However, the use is not exclusive. Specifically, the argument is advanced on the assumption that CSI-RS may be used in the use of (1).
FIG. 7 shows a configuration example of a frame of LTE. In LTE, the minimum unit of the frequency scheduling and the adaptive MCS control is defined as Resource Block (RB, hereinafter referred to as RB) in the frequency direction, and Sub-frame in the time direction. In the signal configuration of one sub-frame and RB (hereinafter, this is referred to as 1 RB/Sub-frame) functioning as a resource unit, as shown in a frame in which RB15 of Sub-frame #0 in the figure is enlarged, a control signal and the reference signal RS are allocated from the head of the time axis, and then data are allocated. Here, 1 RB/Sub-frame consists of 12 sub-carriers in the frequency direction, and 14 OFDM symbols in the time direction. The reference signal RS is allocated in a specific OFDM symbol and sub-carrier in 1 RB/Sub-frame. The unit of these OFDM symbol and sub-carrier is called RE (Resource Element).
FIG. 8 is a diagram showing a conventional example of the above-described CSI-RS transmission method corresponding to LTE-A. The example of FIG. 8 shows, as an example of the CSI-RS transmission method for LTE-A, a method of transmitting CSI-RS for 8 antennas by using only specific RB/Sub-frames (for example, see Non-patent Literature 2). As shown in FIG. 8, the method is configured so that CSI-RS (second reference signal) for 8 antennas is transmitted by using only the RB/Sub-frames indicated by the oblique lines, and, in the other RB/Sub-frames, only 4RS (first reference signal) corresponding to 4 antennas for LTE is transmitted. In the signal configuration of 1 RB/Sub-frame, as shown in a frame in which RB12 of Sub-frame #0 in the figure is enlarged, a control signal and the reference signal RS for LTE are allocated from the head of the time axis, and then CSI-RS for 8 antennas and data are allocated together with the reference signal RS for LTE. In this case, CSI-RS has a form in which RE for data is replaced.
The CSI-RS transmission method is configured so that an LTE user equipment that can compatible only with LTE can receive data in the latter resource in which CSI-RS is not transmitted, and 4RS for LTE is transmitted also in the former resource (RB/Sub-frames of the oblique lines) in which CSI-RS is transmitted, thereby enabling also the LTE user equipment to measure CQI. Furthermore, RB/Sub-frames for transmitting CSI-RS for 8 antennas are discretely allocated. However, it is possible to accurately measure CQI in each resource by interpolating/averaging the resources.